added additional grammars for insert ignore and replace

Not sure if this is something you might consider, but I regularly make use of insert ignore, and replace queries. Insert ignore is great for atomic "does it exist, if not insert it" use cases:

~~~
$id = $qb->table('files')->insertIgnore(array(
    'filename' => '/new/file/name.php'
));

if(!$id)
{
     echo "File exists";
}
~~~

I've added tests, but I've touched half a dozen files so you'll probably want to examine for yourself. No worries if this is not something you want, I just thought I would enquire :)

Cheers for all the hard work, it's a great library!